The present invention pertains to producing paving material. Specifically, the invention relates to drum mixers which can be used to prepare asphalt material for paving.
In recent years, drum mixers have been accepted by the asphalt paving industry over the conventional batch plant mixers because of several advantages. Those advantages include lower equipment costs, a reduced amount of equipment necessary, and the ability to recycle asphalt paving materials. Unfortunately, drum mixers have a significant drawback, i.e. both liquid asphalt and asphalt-coated reclaimed material are exposed to high-temperature gases used for drying the aggregate in the drum mixer. While these gases are necessary to heat and dry the paving material, a "blue smoke" of hydrocarbon gases is produced from exposure of asphaltic material to high-temperature gases. With the advent of state and federal environmental pollution laws, use of the drum mixer requires strict control of emissions of this blue smoke.
In the past, elaborate techniques were utilized to reduce the gas temperature at the point where the liquid asphalt and asphalt-coated materials are introduced into the drum mixer. However, these methods were counter-productive to the purposes for which the gas is used in the drum mixer, i.e. the drying of the aggregate. This produced a slower, unacceptable production rate. Also, the mix itself, when hot enough to use for paving, will give off a small amount of blue smoke.
More recently, the drum mixer industry has attempted to deal with the blue smoke problem by removing the liquid asphalt either from direct addition to an area in the drum exposed to the hot gases, or from the drum itself. Each one of these attempts has been deficient in one way or another.
Standard Havens, Inc.'s new (MAGNUM) hot mix system features a separate chamber that allows the virgin aggregate and reclaimed asphalt pavement to drop out of the hot damaging gas stream before it is mixed with the liquid asphalt additives and fines. While this design concept allegedly eliminates stripping of the light ends that result in blue smoke, the thermal efficiency of this system is low and the exit velocity of the gases is high, causing a problem with very high dust carry out. Astec Industries' coater-mixer has a reclaimed asphalt pavement inlet downstream from the burner and virgin aggregate inlet and adds the liquid asphalt outside the drum. This device removes the liquid asphalt from the hotter gases produced by the burner but utilizes a very expensive mixing chamber or pugmill and does not minimize the exposure of reclaimed asphalt pavement to the hot gases. A third manufacturer, Cedar Rapids, manufactures a "drum in a drum" mixer. In that mixer, virgin aggregate is added in a drum containing the burner and hot gases produced from the burner while the reclaimed asphalt pavement is added initially into a larger drum surrounding the virgin aggregate drum. This mixer exposes the reclaimed asphalt pavement to hot gases and the hot part of the drum for a significant period of time. It should be noted that all three of these commercial drum mixers are parallel flow mixers, i.e. the paving materials travel in the same direction as the direction of flow of the hot gases.
It is an object of the present invention to lessen production of hydrocarbon gases while mixing asphaltic materials inside the drum mixer.
It is another object of the present invention to lessen production of hydrocarbon gases in the drum mixer and to achieve high thermal efficiency.
It is another object of the present invention to incinerate what hydrocarbon gases are unavoidably produced.
It is yet another object of the present invention to achieve counterflow drying of aggregate while removing recycled asphalt pavement and liquid asphalt from direct contact with the radiation and the hot gases produced by the burner.
Other objects, features, and advantages of this invention will be apparent from the following description and illustration of the preferred embodiment of the invention.